Can't get this reduce function to work

Question

Working through Eloquent JavaScript and High Order Functions - section in Functional Programming.

Trying to define a reduce function and use it in a higher order function countWords();, which takes a given array and counts number of times each particular value is present and puts it in an object.

I.e. this works:

function combine(countMap, word) {
    countMap[word] = ++countMap[word] || 1; // made the edit

    return countMap;
}

function countWords(wordArray) {
    return wordArray.reduce(combine, {});
}

var inputWords = ['Apple', 'Banana', 'Apple', 'Pear', 'Pear', 'Pear'];

countWords(inputWords); // {Apple: 2, Banana: 1, Pear: 3}

I.e. this does not:

function combine(countMap, word) {
    countMap[word] = ++countMap[word] || 1;

    return countMap;
}

function forEach(array, action) {
    for (var i = 0; i < array.length; i++) {
        action(array[i]);
    }
}

function reduce(fn, base, array) {
    forEach(array, function (element) {
        base = fn(base, element);
    });

    return base;
}

function countWords(wordArray) {
    return reduce(combine, {}, wordArray);
}

var inputWords = ['Apple', 'Banana', 'Apple', 'Pear', 'Pear', 'Pear'];

countWords(inputWords); //    returned this - [object Object] { ... }  - this is no longer an issue after fix keeping it noted for reference to the original issue.

Any help on this would be great. Thanks.

Answer 1

Your original reduce is actually broken, despite you saying that it works.

Here's a reduce that actually functions

var words = ["foo", "bar", "hello", "world", "foo", "bar"];

var wordIndexer = function(map, word) { 
  map[word] = map[word] || 0;
  map[word]++;
  return map;
};

var count = word.reduce(wordIndexer, {});

console.log(count);

// Object {foo: 2, bar: 2, hello: 1, world: 1}

That said, I'm not entirely sure what you're trying to do with the second half of your post. Are you just trying to write implementations for forEach and reduce so you can understand how they work?

---

I would write the forEach like this

var forEach = function(arr, callback) {
  for (var i=0, len=arr.length; i<len; i++) {
    callback(arr[i], i, arr);
  }
  return arr;
};

And reduce like this

var reduce = function(arr, callback, initialValue) {
  var result = initialValue;
  forEach(arr, function(elem, idx) {
    result = callback(result, elem, idx, arr);
  });
  return result;
};

Test them out

var numbers = [10, 20, 30];

forEach(numbers, function(num, idx) {
  console.log(idx, num);
});

// 0, 10
// 1, 20
// 2, 30
//=> [10, 20, 30]

var n = reduce(numbers, function(sum, num, idx, arr) {
  return sum = sum + num;
}, 0);

console.log(n);
//=> 60

For those curious about reduce callback, I matched the native .reduce callback